Digital computers transfer information in the form of digital pulses to and from a resource memory to which the host computer system has access, whether the memory is located within the system (local) or at a remote location. In processing the data, it is necessary that the timing of these pulses be determined and controlled so that the various logic functions occurring in the host system can be synchronized with the transfer of digital pulses representing digital data contained within the resource memory.
Information, including program instructions as well as other data, is typically stored in a resource memory on a magnetic medium, and each bit of digital data typically appears as a magnetic transition area on the surface of the medium. In order for the host computer to have access to such data, it is important that the bits of data be accurately positioned on the magnetic medium and be capable of being accurately read therefrom. With the growth in the market for personal computers, the use of a particular type of magnetic memory device, the floppy disk, has greatly increased. Furthermore, as personal computers have been redesigned for greater memory capacity and speed, the storage capacity of floppy disks used with these computers has been increased by increasing the density of the digital data recorded on the floppy disk.
The use of disks as storage media is accompanied by certain problems, however, particularly for disk drive systems used in host computer systems available in the less expensive personal computer and business word processor markets. Although it is a design criteria of disk drive manufacturers to accurately control the speed of rotation of the disk in order to have a fixed period of disk rotation and thereby maintain a predetermined frequency at which one can read data from the disk, one problem is frequency drift during data transfers due to unsteadiness in motor drive speed. This results in frequency error and can cause the data on the disk to be read incorrectly. Another problem is migration of the magnetic transition areas, and thus the data bits on the storage medium, due to the inherent characteristics of the magnetic disk. This results in phase error during data transfers and can also result in incorrect data reads. These sources of error are present in every disk drive system to a certain extent, and are addressed by the circuitry of the present invention.